Direct and selective production of ethanol from acetic acid utilizing a platinum/tin catalyst

ABSTRACT

A process for the selective production of ethanol by vapor phase reaction of acetic acid over a hydrogenating catalyst composition to form ethanol is disclosed and claimed. In an embodiment of this invention reaction of acetic acid and hydrogen over a platinum and tin supported on silica, graphite, calcium silicate or silica-alumina selectively produces ethanol in a vapor phase at a temperature of about 250° C.

FIELD OF THE INVENTION

The present invention relates generally to a process for the productionof ethanol from acetic acid. More specifically, the present inventionrelates to a process including hydrogenating acetic acid utilizing acatalyst composed of platinum and tin supported on a suitable catalystsupport optionally containing one or more additional hydrogenatingmetals to form ethanol with high selectivity.

BACKGROUND

There is a long felt need for an economically viable process to convertacetic acid to ethanol. Ethanol is an important commodity feedstock fora variety of industrial products and is also used as a fuel additivewith gasoline. For instance, ethanol can readily be dehydrated toethylene, which can then be converted to polymer products orsmall-molecule based products for use in coatings, polymer manufactureand so forth. Ethanol is conventionally produced from feedstocks whereprice fluctuations are becoming more significant. That is, fluctuatingnatural gas and crude oil prices contribute to fluctuations in the costof conventionally produced, petroleum, natural gas or corn or otheragricultural product-sourced ethanol, making the need for alternativesources of ethanol all the greater when oil prices and/or agriculturalproduct prices rise.

It has been reported that ethanol can be produced from the hydrogenationof acetic acid, but most of these processes feature several drawbacksfor a commercial operation. For instance, U.S. Pat. No. 2,607,807discloses that ethanol can be formed from acetic acid over a rutheniumcatalyst at extremely high pressures of 700-950 bars in order to achieveyields of around 88%, whereas low yields of only about 40% are obtainedat pressures of about 200 bar. Nevertheless, both of these conditionsare unacceptable and uneconomical for a commercial operation.

More recently, it has been reported that ethanol can be produced fromhydrogenating acetic acid using a cobalt catalyst again atsuperatmospheric pressures such as about 40 to 120 bar. See, forexample, U.S. Pat. No. 4,517,391 to Shuster et al. However, the onlyexample disclosed therein employs reaction pressure in the range ofabout 300 bar still making this process undesirable for a commercialoperation. In addition, the process calls for a catalyst containing noless than 50 percent cobalt by weight plus one or more members selectedfrom the group consisting of copper, manganese, molybdenum, chromium,and phosphoric acid, thus rendering the process economically non-viable.Although there is a disclosure of use of simple inert catalyst carriersto support the catalyst materials, there is no specific example ofsupported metal catalysts.

U.S. Pat. No. 5,149,680 to Kitson et al. describes a process for thecatalytic hydrogenation of carboxylic acids and their anhydrides toalcohols and/or esters utilizing a platinum group metal alloy catalysts.The catalyst is comprised of an alloy of at least one noble metal ofGroup VIII of the Periodic Table and at least one metal capable ofalloying with the Group VIII noble metal, admixed with a componentcomprising at least one of the metals rhenium, tungsten or molybdenum.Although it has been claimed therein that improved selectivity toalcohols are achieved over the prior art references it was stillreported that 3 to 9 percent of alkanes, such as methane and ethane areformed as by-products during the hydrogenation of acetic acid to ethanolunder their optimal catalyst conditions.

U.S. Pat. No. 4,777,303 to Kitson et al. describes a process for theproductions of alcohols by the hydrogenation of carboxylic acids. Thecatalyst used in this case is a heterogeneous catalyst comprising afirst component which is either molybdenum or tungsten and a secondcomponent which is a noble metal of Group VIII of the Periodic Table ofthe elements, optionally on a support, for example, a high surface areagraphitized carbon. The selectivity to a combined mixture of alcohol andester is reported to be only in the range of about 73 to 87 percent withlow conversion of carboxylic acids at about 16 to 58 percent. Inaddition, no specific example of conversion of acetic acid to ethanol isprovided.

U.S. Pat. No. 4,804,791 to Kitson et al. describes another process forthe production of alcohols by the hydrogenation of carboxylic acids. Inthis process, ethanol is produced from acetic acid or propanol isproduced from propionic acid by contacting either acetic acid orpropionic acid in the vapor phase with hydrogen at elevated temperatureand a pressure in the range from 1 to 150 bar in the presence of acatalyst comprising as essential components (i) a noble metal of GroupVIII of the Periodic Table of the elements, and (ii) rhenium, optionallyon a support, for example a high surface area graphitized carbon. Theconversion of acetic acid to ethanol ranged from 0.6% to 69% withselectivity to ethanol was in the range of about 6% to 97%.

From the foregoing it is apparent that existing processes do not havethe requisite selectivity to ethanol or existing art employs catalysts,which are expensive and/or non-selective for the formation of ethanoland produces undesirable by-products.

SUMMARY OF THE INVENTION

Surprisingly, it has now been unexpectedly found that ethanol can bemade on an industrial scale directly from acetic acid with very highselectivity and yield. More particularly, this invention provides aprocess for the selective formation of ethanol from acetic acidcomprising: hydrogenating acetic acid over a platinum/tin hydrogenatingcatalyst in the presence of hydrogen. More specifically, the catalystsuitable for the process of this invention is comprised of a combinationof platinum and tin supported on a suitable catalyst support optionallyin combination with one or more metal catalysts selected from the groupconsisting of palladium, rhodium, ruthenium, rhenium, iridium, chromium,copper, molybdenum, tungsten, vanadium and zinc. Suitable catalystsupports include without any limitation, silica, alumina, calciumsilicate, carbon, zirconia and titania.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described in detail below with reference to numerousembodiments for purposes of exemplification and illustration only.Modifications to particular embodiments within the spirit and scope ofthe present invention, set forth in the appended claims, will be readilyapparent to those of skill in the art.

Unless more specifically defined below, terminology as used herein isgiven its ordinary meaning. Mole percent (mole % or %) and like termsrefer to mole percent unless otherwise indicated. Weight percent (wt %or %) and like terms refer to weight percent unless otherwise indicated.

“Conversion” is expressed as a mole percentage based on acetic acid inthe feed. The conversion of acetic acid (AcOH) is calculated from gaschromatography (GC) data using the following equation:

${{AcOH}\mspace{14mu}{conversion}\mspace{11mu}(\%)} = {100*\frac{\begin{matrix}{{{mmol}\mspace{14mu}{AcOH}\mspace{14mu}{in}\mspace{14mu}( {{feed}\mspace{14mu}{stream}} )} -} \\{{mmol}\mspace{14mu}{AcOH}\mspace{14mu}{out}\mspace{14mu}( {G\; C} )}\end{matrix}}{{mmol}\mspace{14mu}{AcOH}\mspace{14mu}{in}\mspace{14mu}( {{feed}\mspace{14mu}{stream}} )}}$

“Selectivity” is expressed as a mole percent based on converted aceticacid. For example, if the conversion is 50 mole % and 50 mole % of theconverted acetic acid is converted to ethanol, we refer to the ethanolselectivity as 50%. Selectivity is calculated from gas chromatography(GC) data using the following equation:

${{Selectivity}\mspace{14mu}{to}\mspace{14mu}{{Et}{OH}}\mspace{11mu}(\%)} = {100*\frac{{mmol}\mspace{14mu}{{Et}{OH}}\mspace{14mu}{out}\mspace{14mu}( {G\; C} )}{\begin{matrix}{\frac{{Total}\mspace{14mu}{mmol}\mspace{14mu} C\mspace{14mu}{out}\mspace{14mu}( {G\; C} )}{2} -} \\{{mmol}\mspace{14mu}{AcOH}\mspace{14mu}{out}\mspace{14mu}( {G\; C} )}\end{matrix}}}$

Weight percent of a catalyst metal is based on metal weight and thetotal dry weight of metal and support.

The reaction proceeds in accordance with the following chemicalequation:

In accordance with the invention, conversion of acetic acid to ethanolcan be carried out in a variety of configurations, such as for examplein a single reaction zone which may be a layered fixed bed, if sodesired. An adiabatic reactor could be used, or a shell and tube reactorprovided with a heat transfer medium could be used. The fixed bed cancomprise a mixture of different catalyst particles or catalyst particleswhich include multiple catalysts as further described herein. The fixedbed may also include a layer of particulate material making up a mixingzone for the reactants. A reaction mixture including acetic acid,hydrogen and optionally an inert carrier gas is fed to the bed as astream under pressure to the mixing zone. The stream is subsequentlysupplied (by way of pressure drop) to the reaction zone or layer.Reaction zone comprises a catalytic composition including a suitablehydrogenating catalyst where acetic acid is hydrogenated to produceethanol. Any suitable particle size may be used depending upon the typeof reactor, throughput requirements and so forth.

Although various platinum containing hydrogenating catalysts known toone skilled in the art can be employed in hydrogenating acetic acid toform ethanol in the process of this invention it is preferred that ahydrogenating catalyst employed contains a combination of platinum andtin on a suitable catalyst support. As noted earlier, it is furtherpreferred that the catalysts that are suitable in the process of thisinvention contain optionally a third metal supported on the samecatalyst support. The following metals may be mentioned as those metalssuitable as a third metal without any limitation: palladium, rhodium,ruthenium, rhenium, iridium, chromium, copper, molybdenum, tungsten,vanadium, zinc and a mixture thereof. Typically, it is preferred that asuitable weight ratio of a combination of platinum and tin on a suitablesupport can be used as a hydrogenating catalyst. Thus a combination ofplatinum and tin (Pt/Sn) in the weight ratio of about 0.1-2 areparticularly preferred. More preferably, a weight ratio of Pt/Sn isabout 0.5-1.5 and most preferably the weight ratio of Pt/Sn is about 1.Preferred examples of metals that can be used with Pt/Sn as a thirdmetal include without any limitation any of the other metals listedabove, such as for example rhodium, iridium, copper, molybdenum andzinc.

Various catalyst supports known in the art can be used to support thecatalysts of this invention. Examples of such supports include withoutany limitation, zeolite, iron oxide, silica, alumina, titania, zirconia,magnesium oxide, calcium silicate, carbon, graphite and a mixturethereof. Preferred supports are silica, alumina, calcium silicate,carbon, zirconia and titania. More preferably silica is used as acatalyst support in the process of this invention. It is also importantto note that higher the purity of silica better it is preferred as asupport in this invention. Another preferred catalyst support is calciumsilicate.

In another embodiment of this invention the preferred catalyst supportis carbon. Various forms of carbon known in the art that are suitable ascatalyst support can be used in the process of this invention.Particularly preferred carbon support is a graphitized carbon,particularly the high surface area graphitized carbon as described inGreat Britain Patent No. 2,136,704. The carbon is preferably inparticulate form, for example, as pellets. The size of the carbonparticles will depend on the pressure drop acceptable in any givenreactor (which gives a minimum pellet size) and reactant diffusionconstraint within the pellet (which gives a maximum pellet size).

The carbon catalyst supports that are suitable in the process of thisinvention are preferably porous carbon catalyst supports. With thepreferred particle sizes the carbon will need to be porous to meet thepreferred surface area characteristics.

The catalyst supports including the carbon catalyst supports may becharacterized by their BET, basal plane, and edge surface areas. The BETsurface area is the surface area determined by nitrogen adsorption usingthe method of Brunauer Emmett and Teller J. Am. Chem. Soc. 60, 309(1938). The basal plane surface area is the surface area determined fromthe heat of adsorption on the carbon of n-dotriacontane from n-heptaneby the method described in Proc. Roy. Soc. A314 pages 473-498, withparticular reference to page 489. The edge surface area is the surfacearea determined from the heat of adsorption on the carbon of n-butanolfrom n-heptane as disclosed in the Proc. Roy. Soc. article mentionedabove with particular reference to page 495.

The preferred carbon catalyst supports for use in the present inventionhave a BET surface area of at least 100 m²/g, more preferably at least200 m²/g, most preferably at least 300 m²/g. The BET surface area ispreferably not greater than 1000 m²/g, more preferably not greater than750 m²/g.

It is preferred to use carbon catalyst supports with ratios of basalplane surface area to edge surface area of at least 10:1, preferably atleast 100:1. It is not believed that there is an upper limit on theratio, although in practice it will not usually exceed 200:1.

The preferred carbon support may be prepared by heat treating acarbon-containing starting material. The starting material may be anoleophillic graphite e.g. prepared as disclosed in Great Britain PatentNo. 1,168,785 or may be a carbon black.

However, oleophillic graphites contain carbon in the form of very fineparticles in flake form and are therefore not very suitable materialsfor use as catalyst supports. We prefer to avoid their use. Similarconsiderations apply to carbon blacks which also have a very fineparticle size.

The preferred materials are activated carbons derived from vegetablematerials e.g. coconut charcoal, or from peat or coal or fromcarbonizable polymers. The materials subjected to the heat treatmentpreferably have particle sizes not less than these indicated above asbeing preferred for the carbon support.

The preferred starting materials have the following characteristics: BETsurface area of at least 100, more preferably at least 500 m²/g.

The preferred heat treatment procedure for preparing carbon supportshaving the defined characteristics, comprise successively (1) heatingthe carbon in an inert atmosphere at a temperature of from 900° C. to3300° C., (2) oxidizing the carbon at a temperature between 300° C. and1200° C., (3) heating in an inert atmosphere at a temperature of between900° C. and 3000° C.

The oxidation step is preferably carried out at temperatures between300° and 600° C. when oxygen (e.g. as air) is used as the oxidizingagent.

The duration of the heating in inert gas is not critical. The timeneeded to heat the carbon to the required maximum temperature issufficient to produce the required changes in the carbon.

The oxidation step must clearly not be carried out under conditions suchthat the carbon combusts completely. It is preferably carried out usinga gaseous oxidizing agent fed at a controlled rate to avoid overoxidation. Examples of gaseous oxidizing agents are steam, carbondioxide, and gases containing molecular oxygen e.g. air. The oxidationis preferably carried out to give a carbon weight loss of at least 10weight percent based on weight of carbon subjected to the oxidationstep, more preferably at least 15 weight percent.

The weight loss is preferably not greater than 40 weight percent of thecarbon subjected to the oxidation step, more preferably not greater than25 weight percent of the carbon.

The rate of supply of oxidizing agent is preferably such that thedesired weight loss takes place over at least 2 hours, more preferablyat least 4 hours.

Where an inert atmosphere is required it may be supplied by nitrogen oran inert gas.

As noted above, the loading levels of platinum and tin are generallyreferenced with the content of platinum and the weight ratio of Pt/Snand is in the range of about 0.1 to 2. Thus, when the weight ratio ofPt/Sn is 0.1, the amount of platinum can be 0.1 or 1 weight percent andthus 1 or 10 weight percent of tin is present on the catalyst support.More preferably, the weight ratio of Pt/Sn is about 0.5, and thus theamount of platinum on the catalyst support can be either 0.5 or 1 weightpercent and that of tin is either one or two weight percent. Morepreferably, the weight ratio of Pt/Sn is one. Thus the amount ofplatinum on a support is 0.5, one or two weight percent and that of tinis also 0.5, one or two weight percent. However, low weight ratios ofPt/Sn can also be employed. For instance, a weight ratio of Pt/Sn of 0.2can also be employed. In such cases, the amount of platinum on thesupport can be 0.5 or one weight percent whereas 2.5 or five weightpercent of tin is employed.

The amount of third metal loading if present on a support is not verycritical in this invention and can vary in the range of about 0.1 weightpercent to about 10 weight percent. A metal loading of about 1 weightpercent to about 6 weight percent based on the weight of the support isparticularly preferred.

The metal impregnation can be carried out using any of the known methodsin the art. Typically, before impregnation the supports are dried at120° C. and shaped to particles having size distribution in the range ofabout 0.2 to 0.4 mm. Optionally the supports may be pressed, crushed andsieved to a desired size distribution. Any of the known methods to shapethe support materials into desired size distribution can be employed.

For supports having low surface area, such as for example alpha-alumina,the metal solutions are added in excess until complete wetness or excessliquid impregnation so as to obtain desirable metal loadings.

As noted above, the hydrogenation catalysts used in the process of thisinvention are at least bimetallic containing platinum and tin.Generally, without intending to be bound by any theory, it is believedthat one metal acts as a promoter metal and the other metal is the mainmetal. For instance, in the instant process of the invention,combination of platinum and tin is considered to be main metal forpreparing hydrogenation catalysts of this invention. However, it canalso be considered that platinum is the main metal and tin is thepromoter metal depending upon various reaction parameters including butnot limited to catalyst support employed, reaction temperature andpressure, etc. The main metal can be combined with a promoter metal suchas tungsten, vanadium, molybdenum, chromium or zinc. However, it shouldbe noted that sometimes main metal can also act as a promoter metal orvice versa. For example, nickel can be used as a promoter metal wheniron is used as a main metal. Similarly, chromium can be used as a mainmetal in conjunction with copper (i.e., Cu—Cr as main bimetallicmetals), which can further be combined with promoter metals such ascerium, magnesium or zinc.

The bimetallic catalysts are generally impregnated in two steps. First,the “promoter” metal is added, followed by “main” metal. Eachimpregnation step is followed by drying and calcination. The bimetalliccatalysts may also be prepared by co-impregnation. For instance, theplatinum/tin catalysts of this invention are generally co-impregnated ona support catalyst. In the case of trimetallic Cu/Cr-containingcatalysts as described above, a sequential impregnation may be used,starting with the addition of the “promoter” metal. The secondimpregnation step may involve co-impregnation of the two principalmetals, i.e., Cu and Cr. For example, Cu—Cr—Co on SiO₂ may be preparedby a first impregnation of chromium nitrate, followed by theco-impregnation of copper and cobalt nitrates. Again, each impregnationis followed by drying and calcinations. In most cases, the impregnationmay be carried out using metal nitrate solutions. However, various othersoluble salts which upon calcination releases metal ions can also beused. Examples of other suitable metal salts for impregnation includemetal oxalate, metal hydroxide, metal oxide, metal acetate, ammoniummetal oxide, such as ammonium heptamolybdate hexahydrate, metal acids,such as perrhenic acid solution, and the like.

Thus in one embodiment of this invention, there is provided ahydrogenation catalyst wherein the catalyst support is graphite with abimetallic loading of platinum and tin. In this aspect of the invention,the loading of platinum is about 0.5 weight percent to about 1 weightpercent and the loading of tin is about 0.5 weight percent to about 5weight percent. Specifically, platinum/tin loading levels of 1/1, 1/5,0.5/0.5, and 0.5/2.5 weight percent on graphite can be used.

In another embodiment of this invention, there is further provided ahydrogenation catalyst wherein the catalyst support is high purity lowsurface area silica with a bimetallic loading of platinum and tin. Inthis aspect of the invention, the loading platinum is about 0.5 weightpercent to about 1 weight percent and the loading of tin is about 0.5weight percent to about 5 weight percent. Specifically, platinum/tinloading levels of 1/1, 1/5, 0.5/0.5, and 0.5/2.5 weight percent on highpurity low surface area silica can be used.

In another embodiment of this invention, there is further provided ahydrogenation catalyst wherein the catalyst support is calcium silicatewith a bimetallic loading of platinum and tin. In this aspect of theinvention, the loading platinum is about 0.5 weight percent to about 1weight percent and the loading of tin is about 0.5 weight percent toabout 5 weight percent. Specifically, platinum/tin loading levels of1/1, 1/5, 0.5/0.5, and 0.5/2.5 weight percent on calcium silicate can beused.

In another embodiment of this invention, there is further provided ahydrogenation catalyst wherein the catalyst support is a silica-aluminawith a bimetallic loading of platinum and tin. In this aspect of theinvention, the loading platinum is about 0.5 weight percent to about 1weight percent and the loading of tin is about 0.5 weight percent toabout 5 weight percent. Specifically, platinum/tin loading levels of1/1, 1/5, 0.5/0.5, and 0.5/2.5 weight percent on calcium silicate can beused.

In general, by the practice of this invention acetic acid canselectivity be converted to ethanol at very high rates. The selectivityto ethanol in general is very high and may be at least 60 percent. Underpreferred reaction conditions, acetic acid is selectively converted toethanol at a selectivity of at least 80 percent or more preferably at aselectivity of at least 90 percent. Most preferably ethanol selectivityis at least 95 percent.

The conversion of acetic acid using the catalysts of this invention isat least 60% with selectivity to ethanol at least 80%, preferably 90%and most preferably 95%.

Generally, the active catalysts of the invention are the non-promotedcatalysts containing platinum and tin supported on silica with platinumand tin loadings of 1 weight percent each. In accordance with thepractice of this invention, acetic acid can be converted using thesecatalysts at conversions of around 90% with ethanol selectivity of atleast 90%, more preferably selectivity to ethanol of at least 95%.

Similar conversions and selectivities are achieved using calciumsilicate, graphite or silica-alumina as a support and with loadings ofplatinum and tin of one weight percent each and with no other promotermetals.

In another aspect of this invention it is also possible to obtain highlevels of conversions in the order of at least 90% and high selectivityto ethanol of at least about 90% using platinum and tin loadings of oneweight percent each on silica or calcium silicate as catalyst supportswith a promoter metal, such as for example cobalt, ruthenium orpalladium. The promoter metal loadings is in the range of about 0.1weight percent to about 0.5 weight percent, more preferably in the rangeof about 0.15 weight percent to 0.3 weight percent and most preferablythe promoter metal loading is about 0.2 weight percent. In this aspectof the invention, other preferred catalyst supports includesilica-alumina, titania or zirconia.

In another aspect of the process of this invention, the hydrogenation iscarried out at a pressure just sufficient to overcome the pressure dropacross the catalytic bed.

The reaction may be carried out in the vapor or liquid state under awide variety of conditions. Preferably, the reaction is carried out inthe vapor phase. Reaction temperatures may be employed, for example inthe range of about 200° C. to about 300° C., preferably about 225° C. toabout 275° C. The pressure is generally uncritical to the reaction andsubatmospheric, atmospheric or superatmospheric pressures may beemployed. In most cases, however, the pressure of the reaction will bein the range of about 1 to 30 atmospheres absolute, most preferably thepressure of reaction zone is in the range of about 10 to 25 atmospheresabsolute.

Although the reaction consumes two moles of hydrogen per mole of aceticacid to produce a mole of ethanol, the actual molar ratio of acetic acidto hydrogen in the feed stream may be varied between wide limits, e.g.from about 100:1 to 1:100. It is preferred however that such ratio be inthe range of about 1:20 to 1:2. More preferably the molar ratio ofacetic acid to hydrogen is about 1:5.

The raw materials used in connection with the process of this inventionmay be derived from any suitable source including natural gas,petroleum, coal, biomass and so forth. It is well known to produceacetic acid through methanol carbonylation, acetaldehyde oxidation,ethylene oxidation, oxidative fermentation, and anaerobic fermentationand so forth. As petroleum and natural gas have become more expensive,methods for producing acetic acid and intermediates such as methanol andcarbon monoxide from alternate carbon sources have drawn more interest.Of particular interest is the production of acetic acid from synthesisgas (syngas) that may be derived from any suitable carbon source. U.S.Pat. No. 6,232,352 to Vidalin, the disclosure of which is incorporatedherein by reference, for example, teaches a method of retrofitting amethanol plant for the manufacture of acetic acid. By retrofitting amethanol plant the large capital costs associated with CO generation fora new acetic acid plant are significantly reduced or largely eliminated.All or part of the syngas is diverted from the methanol synthesis loopand supplied to a separator unit to recover CO and hydrogen, which arethen used to produce acetic acid. In addition to acetic acid, theprocess can also be used to make hydrogen which is utilized inconnection with this invention.

U.S. Pat. No. RE 35,377 Steinberg et al., also incorporated herein byreference, provides a method for the production of methanol byconversion of carbonaceous materials such as oil, coal, natural gas andbiomass materials. The process includes hydrogasification of solidand/or liquid carbonaceous materials to obtain a process gas which issteam pyrolized with additional natural gas to form synthesis gas. Thesyngas is converted to methanol which may be carbonylated to aceticacid. The method likewise produces hydrogen which may be used inconnection with this invention as noted above. See also, U.S. Pat. No.5,821,111 Grady et al., which discloses a process for converting wastebiomass through gasification into synthesis gas as well as U.S. Pat. No.6,685,754 Kindig et al., the disclosures of which are incorporatedherein by reference.

The acetic acid may be vaporized at the reaction temperature, and thenit can be fed along with hydrogen in undiluted state or diluted with arelatively inert carrier gas, such as nitrogen, argon, helium, carbondioxide and the like.

Alternatively, acetic acid in vapor form may be taken directly as crudeproduct from the flash vessel of a methanol carbonylation unit of theclass described in U.S. Pat. No. 6,657,078 of Scates et al., thedisclosure of which is incorporated herein by reference. The crude vaporproduct may be fed directly to the reaction zones of the presentinvention without the need for condensing the acetic acid and light endsor removing water, saving overall processing costs.

Contact or residence time can also vary widely, depending upon suchvariables as amount of acetic acid, catalyst, reactor, temperature andpressure. Typical contact times range from a fraction of a second tomore than several hours when a catalyst system other than a fixed bed isused, with preferred contact times, at least for vapor phase reactions,between about 0.5 and 100 seconds.

Typically, the catalyst is employed in a fixed bed reactor e.g. in theshape of an elongated pipe or tube where the reactants, typically in thevapor form, are passed over or through the catalyst. Other reactors,such as fluid or ebullient bed reactors, can be employed, if desired. Insome instances, it is advantageous to use the hydrogenation catalysts inconjunction with an inert material to regulate the pressure drop, flow,heat balance or other process parameters in the catalyst bed includingthe contact time of the reactant compounds with the catalyst particles.

In one of the preferred embodiments there is also provided a process forselective and direct formation of ethanol from acetic acid comprising:contacting a feed stream containing acetic acid and hydrogen at anelevated temperature with a suitable hydrogenating catalyst containingabout 0.5 weight percent to about 1 weight percent of platinum and about0.5 weight percent to about 5 weight percent of tin on a suitablecatalyst support and optionally a third metal supported on said supportand wherein said third metal is selected from the group consisting ofcobalt, ruthenium and palladium.

In this embodiment of the process of this invention, the preferredhydrogenation catalyst contains about one (1) weight percent platinumand about one (1) weight percent tin. In this embodiment of the processof this invention it is preferred that the hydrogenation catalysts islayered in a fixed bed and the reaction is carried out in the vaporphase using a feed stream of acetic acid and hydrogen in the molar rangeof about 1:20 to 1:5 and at a temperature in the range of about 225° C.to 275° C. and at a pressure of reaction zones in the range of about 10to 25 atmospheres absolute, and the contact time of reactants is in therange of about 0.5 and 100 seconds.

The following examples describe the procedures used for the preparationof various catalysts employed in the process of this invention.

Example A Preparation of 1 Weight Percent Platinum and 1 Weight PercentTin on Graphite

Powdered and meshed graphite (100 g) of uniform particle sizedistribution of about 0.2 mm was dried at 120° C. in an oven undernitrogen atmosphere overnight and then cooled to room temperature. Tothis was added a solution of platinum nitrate (Chempur) (1.64 g) indistilled water (16 ml) and a solution of tin oxalate (Alfa Aesar) (1.74g) in dilute nitric acid (1N, 8.5 ml). The resulting slurry was dried inan oven gradually heated to 110° C. (>2 hours, 10° C./min.). Theimpregnated catalyst mixture was then calcined at 400° C. (6 hours, 1°C./min).

Examples B Preparation of 0.5 Weight Percent Platinum and 5 WeightPercent Tin on High Purity Low Surface Area Silica

Powdered and meshed high purity low surface area silica (100 g) ofuniform particle size distribution of about 0.2 mm was dried at 120° C.in an oven under nitrogen atmosphere overnight and then cooled to roomtemperature. To this was added a solution of platinum nitrate (Chempur)(0.82 g) in distilled water (8 ml) and a solution of tin oxalate (AlfaAesar) (8.7 g) in dilute nitric acid (1N, 43.5 ml). The resulting slurrywas dried in an oven gradually heated to 110° C. (>2 hours, 10°C./min.). The impregnated catalyst mixture was then calcined at 500° C.(6 hours, 1° C./min).

Example C Preparation of 1 Weight Percent Platinum and 1 Weight PercentTin on High Purity Low Surface Area Silica

The procedures of Example B was substantially repeated except forutilizing a solution of platinum nitrate (Chempur) (1.64 g) in distilledwater (16 ml) and a solution of tin oxalate (Alfa Aesar) (1.74 g) indilute nitric acid 1N, 8.5 ml).

Example D Preparation of 1 Weight Percent Platinum and 1 Weight PercentTin on Calcium Silicate

The procedures of Example B was substantially repeated except forutilizing a solution of platinum nitrate (Chempur) (1.64 g) in distilledwater (16 ml) and a solution of tin oxalate (Alfa Aesar) (1.74 g) indilute nitric acid (1N, 8.5 ml), and utilizing calcium silicate as acatalyst support.

Example E Preparation of 0.5 Weight Percent Platinum, 0.5 Weight PercentTin and 0.2 Weight Percent Cobalt on High Purity Low Surface Area Silica

Powdered and meshed high purity low surface area silica (100 g) ofuniform particle size distribution of about 0.2 mm was dried at 120° C.in an oven under nitrogen atmosphere overnight and then cooled to roomtemperature. To this was added a solution of platinum nitrate (Chempur)(0.82 g) in distilled water (8 ml) and a solution of tin oxalate (AlfaAesar) (0.87 g) in dilute nitric acid (1N, 4.5 ml). The resulting slurrywas dried in an oven gradually heated to 110° C. (>2 hours, 10°C./min.). The impregnated catalyst mixture was then calcined at 500° C.(6 hours, 1° C./min). To this calcined and cooled material was added asolution of cobalt nitrate hexahydrate (0.99 g) in distilled water (2ml). The resulting slurry was dried in an oven gradually heated to 110°C. (>2 hours, 10° C./min.). The impregnated catalyst mixture was thencalcined at 500° C. (6 hours, 1° C./min).

Example F Preparation of 0.5 Weight Percent Tin on High Purity LowSurface Area Silica

Powdered and meshed high purity low surface area silica (100 g) ofuniform particle size distribution of about 0.2 mm was dried at 120° C.in an oven under nitrogen atmosphere overnight and then cooled to roomtemperature. To this was added a solution of tin oxalate (Alfa Aesar)(1.74 g) in dilute nitric acid (1N, 8.5 ml). The resulting slurry wasdried in an oven gradually heated to 110° C. (>2 hours, 10° C./min.).The impregnated catalyst mixture was then calcined at 500° C. (6 hours,1° C./min).

Gas Chromatographic (GC) Analysis of the Products

The analysis of the products was carried out by online GC. A threechannel compact GC equipped with one flame ionization detector (FID) and2 thermal conducting detectors (TCDs) was used to analyze the reactantsand products. The front channel was equipped with an FID and a CP-Sil 5(20 m)+WaxFFap (5 m) column and was used to quantify:

Acetaldehyde

Ethanol

Acetone

Methyl acetate

Vinyl acetate

Ethyl acetate

Acetic acid

Ethylene glycol diacetate

Ethylene glycol

Ethylidene diacetate

Paraldehyde

The middle channel was equipped with a TCD and Porabond Q column and wasused to quantify:

CO₂

Ethylene

Ethane

The back channel was equipped with a TCD and Molsieve 5A column and wasused to quantify:

Helium

Hydrogen

Nitrogen

Methane

Carbon monoxide

Prior to reactions, the retention time of the different components wasdetermined by spiking with individual compounds and the GCs werecalibrated either with a calibration gas of known composition or withliquid solutions of known compositions. This allowed the determinationof the response factors for the various components.

Example 1

The catalyst utilized was 1 weight percent platinum and 1 weight percenttin on silica prepared in accordance with the procedure of Example C.

In a tubular reactor made of stainless steel, having an internaldiameter of 30 mm and capable of being raised to a controlledtemperature, there are arranged 50 ml of 1 weight percent platinum and 1weight percent tin on silica. The length of the catalyst bed aftercharging was approximately about 70 mm.

A feed liquid was comprised essentially of acetic acid. The reactionfeed liquid was evaporated and charged to the reactor along withhydrogen and helium as a carrier gas with an average combined gas hourlyspace velocity (GHSV) of about 2500 hr⁻¹ at a temperature of about 250°C. and pressure of 22 bar. The resulting feed stream contained a molepercent of acetic acid from about 4.4% to about 13.8% and the molepercent of hydrogen from about 14% to about 77%. A portion of the vaporeffluent was passed through a gas chromatograph for analysis of thecontents of the effluents. The selectivity to ethanol was 93.4% at aconversion of acetic acid of 85%.

Example 2

The catalyst utilized was 1 weight percent platinum and 1 weight percenttin on calcium silicate prepared in accordance with the procedure ofExample D.

The procedure as set forth in Example 1 is substantially repeated withan average combined gas hourly space velocity (GHSV) of 2,500 hr⁻¹ ofthe feed stream of the vaporized acetic acid and hydrogen at atemperature of 250° C. and pressure of 22 bar. A portion of the vaporeffluent is passed through a gas chromatograph for analysis of thecontents of the effluents. The acetic acid conversion is greater than70% and ethanol selectivity is 99%.

Comparative Example

The catalyst utilized was 1 weight percent tin on low surface area highpurity silica prepared in accordance with the procedure of Example F.

The procedure as set forth in Example 1 is substantially repeated withan average combined gas hourly space velocity (GHSV) of 2,500 hr⁻¹ ofthe feed stream of the vaporized acetic acid and hydrogen at atemperature of 250° C. and pressure of 22 bar. A portion of the vaporeffluent is passed through a gas chromatograph for analysis of thecontents of the effluents. The acetic acid conversion is less than 10%and ethanol selectivity is less than 1%.

While the invention has been illustrated in connection with particularexamples, modifications to these examples within the spirit and scope ofthe invention will be readily apparent to those of skill in the art. Inview of the foregoing discussion, relevant knowledge in the art andreferences discussed above in connection with the Background andDetailed Description, the disclosures of which are all incorporatedherein by reference, further description is deemed unnecessary.

What is claimed is:
 1. A process for selective and direct formation ofethanol from acetic acid comprising: contacting a feed stream containingacetic acid and hydrogen in vapor form at an elevated temperature with ahydrogenating catalyst consisting essentially of platinum and tin on acatalyst support selected from the group consisting of silica, alumina,silica-alumina, calcium silicate, carbon, zirconia, titania andcombinations thereof, wherein at least about 80% by weight of the aceticacid consumed is converted to ethanol.
 2. The process according to claim1, wherein the catalyst contains platinum and tin of from about 0.5weight percent to about 1 weight percent of platinum and from about 0.5weight percent to about 5 weight percent of tin.
 3. The processaccording to claim 1, wherein the catalyst contains platinum and tin ata Pt/Sn weight ratio in the range of about 0.1 to about
 2. 4. Theprocess according to claim 1, wherein the catalyst contains platinum andtin at a Pt/Sn weight ratio in the range of about 0.5 to about
 2. 5. Theprocess according to claim 1, wherein the catalyst contains platinum andtin at a Pt/Sn weight ratio in the range of about 1 to about
 2. 6. Theprocess according to claim 1, wherein the catalyst support is calciumsilicate.
 7. The process according to claim 1, wherein the selectivityto ethanol based on acetic acid consumed is at least 90 percent.
 8. Theprocess according to claim 1, wherein the hydrogenation to ethanol iscarried out in the vapor phase and at a temperature in the range ofabout 200° to 300° C.
 9. The process according to claim 1, wherein thehydrogenation to ethanol is carried out in the vapor phase and at atemperature in the range of about 225° to 275° C.
 10. The processaccording to claim 1, wherein the pressure of reaction zones is in therange of about 1 to 30 atmospheres absolute.
 11. The process accordingto claim 1, wherein the pressure of reaction zones is in the range ofabout 10 to 25 atmospheres absolute.
 12. The process according to claim1, wherein the reactants consist of acetic acid and hydrogen with amolar ratio in the range of about 1:20 to 1:2.
 13. A process forselective and direct formation of ethanol from acetic acid comprising:contacting a feed stream containing acetic acid and hydrogen in vaporform at an elevated temperature with a hydrogenating catalyst consistingessentially of platinum and tin on a catalyst support, wherein thecatalyst contains from about 0.5 weight percent to about 1 weightpercent of platinum.
 14. The process according to claim 13, wherein thecatalyst support is selected from the group consisting of silica,alumina, silica-alumina, calcium silicate, carbon, zirconia, titania andcombinations thereof.
 15. The process according to claim 13, wherein thecatalyst contains from about 0.5 weight percent to about 5 weightpercent of tin.
 16. The process according to claim 13, wherein thecatalyst contains platinum and tin at a Pt/Sn weight ratio in the rangeof about 1 to about 2.